1. Field of the Invention
The present invention relates generally to the field of optical surveying instruments, and specifically to the field of differentiation of targets in optical station.
2. Description of the Background Art
In the available art related to optical surveying instruments, there are optical surveying instruments designed to work in areas where the GPS signals are obstructed or otherwise unavailable, such as tunnels, parking garages, and dense forests as well as to measure building facades or dam faces that are difficult or dangerous to reach.
Trimble Navigation Ltd, based in Sunnyvale, Calif., introduced in 1998 the TTS(trademark) 500 optical surveying instrument that is designed to work in the areas where satellite signals are unavailable. The TTS 500 system incorporates a unique Electronic Distance Meter (EDM) design and a high quality optical system. This design enables high-speed, high accuracy distance measurements to almost any object, as well as to both reflective sheet targets and traditional glass prisms.
The TTS 500 system utilizes the precise measurement of timing information in order to calculate a range measurement. The TTS 500 system calculates distance measurements through the measurement of the travel time or xe2x80x9ctime of flightxe2x80x9d of short duration light pulses. The TTS 500 system generates short infrared light pulses which are transmitted through the telescope to a selected target, such a concrete wall or cube corner reflector (a traditional survey prism reflector). The light pulses reflect off the target and returned to the TTS 500 where they are focused by the receiver lens onto a high-speed light detector within the instrument. The TTS electronics determine the round time for the light pulse. The travel time is used to compute the distance between the instrument and the target. The method of using the time of flight differs from the traditional phase resolving technique typically used in optical total stations for survey applications. A detailed discussion of both the traditional and time of flight methods can be found in J. M. Rxc3xceger""s book, Electronic Distance measurements, An Introduction, Fourth Edition, Springer-Verlag, Berlin (1996).
The light source of the Laser Module is a pulsed laser diode. The pulsed laser in the Laser Module generates a short infrared light pulse, which is optically divided into two parts. The EDM incorporates a 30xc3x97-magnification telescope with integrated cross-hairs (reticle) and an internal focusing mechanism. The main part of the Laser Pulse passes through the telescope, which is aimed at a distant target. A prism-based beam splitter is integrated into the telescope, which allows the infrared laser beam to be aligned with the optical path for visible light. The beam splitter uses a dichroic 45xc2x0 mirror built into the prism. A dichroic (two-color) mirror has different behavior at different wavelengths. The mirror does not reflect visible light and allows it to pass from the target through the telescope to the operator""s eyepiece. However, the 45xc2x0 mirror reflects the infrared radiation of the laser. A small fraction of the light pulse, the Start Pulse, is received by a photosensitive diode in the Start Pulse receiver. The Start Pulse signal initiates a timing process using a high precision clock. The light from the transmitted Laser Pulse is reflected from the target""s surface. A second photosensitive receiver detector in the EDM receives a fraction of the reflected light. This signal is called the Echo Pulse and stops the clock""s timing process.
The distance between the ITS 500 EDM and various targets can be measured over a range from 2 meters to more than 10,000 meters. The distance is dependent on acceptable atmospheric conditions. With the TTS 500 EDM distances can be measured from natural surfaces, reflective surfaces such as foils and bicycle reflectors, and conventional survey glass prisms. The practical operating range of the TTS 500 EDM depends on several factors: (a) the energy of the laser pulse; (b) the reflectivity of the target; (c) the target distance; (d) the sensitivity of the receiver; and (e) possible interference from external noise sources such as sunlight and atmospheric effects that absorb or scatter the light.
The detailed measurements of the Start Pulse and Echo Pulse timing, and the internal processes to initiate measurements and report them to the instrument""s""s computer, are carried out in an on-board signal processing system within the EDM. The TTS 500 EDM signal processing system includes dedicated custom integrated circuits that are designed to manage very fast signals. The TTS 500 EDM electronics system, with its corrections, can measure the target distance with a single measurement to within 5 centimeters. In addition, the EDM uses statistical signal processing to reduce the measurement uncertainty to less than 5 millimeters. The user can select the number of pulses in the statistical computation. The Laser Pulse repetition rate is 1000 pulses per second. The most accurate measurements average the results of 1024 pulses.
The TTS 500 EDM system measures distances till targets that can vary from dark natural surfaces to reflective sheets and polished cube corner reflectors. The variations in target reflectivity, target distance, and atmospheric turbulence, creates significant variations in the amplitude of the reflected echo signal. The total dynamic range of the signal amplitude that should be measured is approximately 1 to 1 million. Thus, the TTS 500 EDM has the dynamic range to measure to both cooperative targets and non-cooperative targets. Cooperative targets are those that have strong reflectivity in a direction back to the source of illumination. These include reflective sheets, bicycle reflectors, and cube corner prisms. The EDM can also measure distances to non-cooperative targets such as concrete walls, paper targets, and mounts of crushed rock piles.
The operator can select the EDM sensitivity. There are two modes of operation: High Sensitivity and Reduced Sensitivity. In either mode, the Laser Pulse energy is the same, while the Echo Pulse receiver sensitivity is changed by a factor of approximately 100. The High Sensitivity mode is particularly useful in situations when the target is not readily accessible. This may be due to difficult or dangerous to reach locations, for example high walls or building faces. The High Sensitivity mode also allows for long distance prism readings. The Reduced Sensitivity mode is ideal for measurements from nearby structures and targets where other sources of reflected light might interfere with the signal. The Reduced Sensitivity mode ensures the signal from the target is the preferred signal.
A typical prism might reflect 95% of the light that falls on it. On the other hand, a non-reflective target can reflect between 90% (for a white painted smooth wall) and 5% (for a black painted surface) of the light. However, even distances to targets with low reflectivity (non-cooperative targets) can be accurately measured using the TTS 500 EDM system. Indeed, when there is a significant attenuation of the signal, such as in the case of a non-cooperative target, the pulsed laser signal (as opposed to the modulated light wave) can successfully overcome the noise within the instrument, because the pulsed laser signal has the peak power 100,000 times greater than the average power of the modulated light wave used in the phase resolving method. This characteristic enables the accurate measurements to targets with low reflectivity. Table 1 and Table 2 summarize the range specifications for the two sensitivity modes.
The problem with the prior art TTS 500 system is that it cannot differentiate very well between a cooperative target placed far away from the EDM, and a non-cooperative target placed close enough to the EDM, because the amplitudes of the reflected signals from both the far-away cooperative target and the close-enough non-cooperative target can be very close. To deal with this problem, the prior art EDM can support the xe2x80x9ccooperative target-onlyxe2x80x9d mode (or xe2x80x9cprism-onlyxe2x80x9d mode) by turning down the sensitivity of the EDM to filter out the weaker return signals. This means that the surveyor would give up range to a located far away cooperative target when in the xe2x80x9cprism-onlyxe2x80x9d mode, but still would be able to see and measure distances to a close enough (within 30 meters of the EDM) non-cooperative target if it is a reasonable good reflector. So, the prior art EDM design gives up the range to a desirable, but located far away cooperative target, and still cannot filter out an undesirable, but located close enough non-cooperative target. There is therefore a need in the art for an efficient solution to this challenge.
To address the shortcomings of the available art, the present invention provides a method and a system for selecting at least one cooperative target from a set of targets, wherein the set of targets includes a subset of cooperative targets and a subset of non-cooperative targets.
In one embodiment, the method of the present invention comprises the following steps: (a) generating an optical signal by using an Optical Station; (b) directing the optical signal at each target from the set of targets; (c) receiving at least one reflected optical signal by the Optical Station, wherein each reflected optical signal is reflected by one target from the set of targets, and wherein each target is marked by the reflection; (d) matching each reflected optical signal with one target from the set of targets; and (e) analyzing at least one reflected signal in order to select at least one cooperative target from the set of all targets.
In another embodiment of the present invention, an empirically generated look-up table that relates an expected amplitude of a reflected signal from a cooperative target to a distance from the cooperative target to the Optical Station can be used to select a target from the set of targets. A target is selected only if an amplitude of an actual reflected signal from the selected target is equal to or greater than an expected amplitude of a reflected signal from a cooperative target placed at the selected target""s distance. If this is the case, the selected target comprises a cooperative target.
In one more embodiment of the present invention, a database comprising a set of elements is created. Each element of the database includes a distance from the Optical Station to a target from the set of targets and an amplitude of an actual signal reflected from that target. The database is post processed to create a plurality of colors, wherein each color is a function of one element of the database. The plurality of colors can be used to create a color- image of each target.